Several techniques have been used to sense and measure the size, distance, geometry, spatial relationship and other parameters of remotely positioned objects. Such techniques include: 1) television sensors operating in the visible or near infrared parts of the electromagnetic spectrum: 2) imaging sensors utilizing CCD arrays: 3) acoustic ranging sensors; and 4) radar systems operating in a variety of different wavelengths. There have been attempts to use these techniques to address the need for accurate measurement of certain object parameters on a stationary or semi-continuous platform that is remotely positioned from the measuring system. One of the hallmarks of this methodology is the requirement for precise analysis of the size, shape and geometry of objects on the platform. Furthermore, in the semi-continuous platform situation or where multiple objects are positioned on a single platform, the spatial relationships between individual objects within the platform field will be required for accurate measurement and parametric assessments. Such measurement and assessments of data has been found useful for example, in manufacturing processes, metrology processes, materials handling, automated warehousing, quality control, security, robotic vision, and human vision surgery.
It has been known that due to the relative small wavelengths of lasers the potential exists to use laser technology to perform accurate measurements and detection. Several techniques for measuring distance with laser light signals are in common use today. They can be divided in two general categories, those that use the speed of light in some way to determine distance, and those that do not. The latter group usually uses a light projector located some distance from both the surface being lit and the detector. The detector then measures the direction of the light from both the surface being lit and the detector and triangulation is used to determine the surface's position. The accuracy of these systems depends on the separation distance between the emitter and detector, and typically works over a relatively narrow range of distances.
For two and three dimensional scanning, multiple axis laser systems generally employ a vertical scanning mirror orthogonally positioned with respect to the horizontal scanning mirror.
One laser system used the combination of the vertical scanning mirror and the horizontal scanning mirror, allowing the laser beam to scan an entire area with the intensity-modulated laser beam. Upon reflection from the object, the intensity modulation continued in the reflected beam and as a result, the reflected beam has the same type of intensity modulation as the probe beam, but with a different phase or time due to the additional distance each beam traveled. When the reflected beam returned, the light energy was converted to electrical energy by a photo-detector, and the phase of the intensity-modulated reflected beam was compared with the phase of the reference signal and a distance measurement obtained. However, the accuracy of the phase comparison method is limited by the ability of the phase detector used to resolve phase and the amount of isolation that can be obtained between the incoming and outgoing signals. Higher frequencies improve the resolution but worsen the crosstalk problem. The phase method of ranging has the additional problem that the range reading aliases at range intervals equal to half the wavelength of the modulation. For example, with 50 MHz modulation (6 meter wavelength), it is not possible to distinguish between actual distances of n, n+3, n+6, . . . meters, since all of them will result in the same detected phase difference. Multiple frequencies or some other technique must be used to resolve this ambiguity in many practical applications.
None of the above described systems are entirely satisfactory. It is therefore desirable to provide a laser distance measurement system in which the above described difficulties encountered by the above described systems are overcome.